Remove "control group" concept from the focus model

The control group concept existed to give special focus behavior to
<dialog> elements, treating them on par with Documents in terms of
allowing focus to shift inside of them. However, this was never
implemented in user agents.

This change removes the control group concept, instead using Document
directly. This allows some simplifications, e.g. because the first
focusable area of a non-empty Document is always the Document's
viewport.

The change uncovered some areas of the spec that were potentially wrong
or unclear when considering focus around documents. We will follow up on
that in #3675. But for now we keep
the same algorithms, just without <dialog>s.

Fixes #2171.